Solar System Dwarf Planet "Haumea" Has a Mystery Spot

Haumea, the mini planet whose detection set off an international and as yet unresolved war of words in 2005 between the two teams claiming its discovery, is back on the astronomy scene with more intrigue.

More recently, observations performed by Lacerda and his colleagues in 2007 revealed a telltale variation in the dwarf planet's reflected brightness as it spins. "The object is egg-shaped, so when you see it sideways it reflects more sunlight than when you see it tip-on," Lacerda says. But the peak reflectance varies from one side to the other, revealing the presence of a dark spot on the dimmer side. "It's as if you were looking at a football that is white, but one of the sides has a spot on it, so it looks darker overall," Lacerda explains.

What is more, the spot reflects more red light than blue light, meaning it has a reddish tint in the visible spectrum. That tint could result from a local concentration of red-reflecting organic compounds or blue-absorbing minerals on Haumea's icy surface.

In January, Lacerda published the results of an infrared survey of Haumea to build on the optical studies that turned up the dark red spot. That second examination turned up a variation in the reflectance band associated with water ice, thought to make up the bulk of the Haumean surface, at the mysterious surface feature. That variation could be explained, Lacerda says, by ice in crystalline, rather than amorphous, form.

Haumea resides far away in the solar system in a region called the Kuiper belt, a ring of icy bodies that extends roughly from the orbital path of Neptune out to the farthest reach of Pluto's orbit. In that distant region, Lacerda explains, ice usually takes the amorphous form, a random molecular arrangement that occurs during rapid freezing in extremely low temperatures. But if ice were temporarily heated and allowed to refreeze, it could take its more ordered crystalline structure.

"The fact that I think I find more crystalline water on the spot means that the temperature on the spot may have been a little bit higher in the past, so it may have been heated right there," Lacerda says. The source of the heat could have been the impact of a small object—perhaps a dark, reddish one bearing organic molecules—which could explain many of the spot's features, as could an impactor that excavated Haumea's inner minerals, the contents of which are unknown.

If the spot indeed proves to mark an impact crater, astronomers could be afforded a peek into the dwarf planet's interior. But Lacerda acknowledges that crystalline water is only one possible interpretation of the infrared data. "It's all very speculative," he says.

"The problem is we don't know how big the spot is," Lacerda says. Haumea is so far away—farther than Pluto, even—that its features cannot be resolved in any detail. Astronomers have to rely on its bulk properties to make inferences about its specifics, in this case tracking the total amount of light Haumea reflects to uncover details about the spot. "It could be very big and not very different in color, just slightly red and slightly darker than the object, or it could be much smaller but much redder and much darker."

Lacerda has booked time next March on the ESO Very Large Telescope (VLT) in Chile with hopes of unraveling some of the mysteries of Haumea and its spot. Using the VLT, he and his colleagues will have higher-resolution measurements of the dwarf planet's light spectrum, which could help narrow the field of possible explanations. "That's the exciting thing," Lacerda says. "Maybe we'll be able to tell what the spot is really made of."